Correct isSpecifiedScalarType refinement, fix #2153

Author: tgriesser

tstypes/type/scalars.d.ts

@@ -1,11 +1,18 @@
-import { GraphQLScalarType } from './definition';
+import { GraphQLScalarType, isScalarType } from './definition';
 
-export const GraphQLInt: GraphQLScalarType;
-export const GraphQLFloat: GraphQLScalarType;
-export const GraphQLString: GraphQLScalarType;
-export const GraphQLBoolean: GraphQLScalarType;
-export const GraphQLID: GraphQLScalarType;
+export const GraphQLInt: GraphQLScalarType & { name: 'Int' };
+export const GraphQLFloat: GraphQLScalarType & { name: 'Float' };
+export const GraphQLString: GraphQLScalarType & { name: 'String' };
+export const GraphQLBoolean: GraphQLScalarType & { name: 'Boolean' };
+export const GraphQLID: GraphQLScalarType & { name: 'ID' };
+
+export type SpecifiedScalarType =
+  | typeof GraphQLInt
+  | typeof GraphQLFloat
+  | typeof GraphQLString
+  | typeof GraphQLBoolean
+  | typeof GraphQLID;
 
 export const specifiedScalarTypes: ReadonlyArray<GraphQLScalarType>;
 
-export function isSpecifiedScalarType(type: any): type is GraphQLScalarType;
+export function isSpecifiedScalarType(type: any): type is SpecifiedScalarType;